In a PCM modem system during a startup mode, the power level of the transmitter of the analog modem is set by estimating at the digital modem what its power level should be. This is accomplished during the startup mode through the specification of mapping parameters and transmit constellation at the digital modem, with the appropriate parameters being transmitted back to the analog modem through the digital network and the analog loop. In addition to the above mapping parameters and transmit constellation, these parameters include pre-equalization parameters which are utilized to set the pre-equalization circuits within the analog modem so as to optimize the PCM modem system.
One way of assigning transmit power levels at the analog modem is to utilize the mapping parameters in which the input data is mapped to a number of equivalence classes. As will be seen, it is the number of equivalence classes which determines the power level of the analog modem transmitter.
By way of background, and as to the start up mode, a set of start up procedures is described in U.S. patent application Ser. No. 09-390,106 entitled “Method and Apparatus for a Start Up Procedure for Digital and Analog Modems Utilizing Pulse Code Modulation for Data Transmission”, filed Sep. 3, 1999, assigned to the assignee here of and incorporated herein by reference.
Inherent in the startup procedure is a routine in which training signals are sent from the analog modem over the upstream channel to the digital modem. The training signals are analyzed at the digital modem so as to be able to characterize the upstream channel. Having measured the upstream channel characteristics and generated transmit constellations to satisfy a certain decoding error probability, the digital modem generates mapping parameters for use with a given transmit constellation. The transmit constellation refers to the specific levels or steps used in the PCM modulation scheme. Given that during startup the digital modem measures upstream channel characteristics, it subsequently estimates the appropriate transmit power level for the analog modem to provide robust communication while satisfying FCC regulations on transmit power.
However, estimating the analog modem transmit power at the digital modem can only result in an approximation of the required transmit level for the transmitter in the analog modem. The estimation is by its very nature inexact and results in non-linearities and downstream performance degradation if the transmit power level is higher than that desired. Moreover, if the transmit power level at the analog modem is too low, the data rate declines.
In addition to non-optimal power levels engendered by the variability of the upstream channel, is also important that the analog modem's transmit power levels be kept within limits set by the Federal Communication Commission. In the present FCC regulation (Part 68) the average power level from the analog modem must be less than −9 dBm. Normally however, the optimal power level for the analog modem transmitter is less than −12 dBm to provide better downstream performance. As the analog modem transmit power increases, downstream higher echo power will be experienced and there will be a larger noise component due to imperfect echo cancellation.
What will be appreciated is that the setting of the analog modem transmitter power level is accomplished remotely from the digital modem. This is done in one embodiment by specifying the number of equivalence classes to which the incoming data is mapped. As will be seen hereinafter, the power level at the analog transmitter is proportional to the number of equivalence classes. Thus, when during startup the upstream channel is measured, the power level estimation is provided through the specification of a trial number of equivalence classes to be used.